This application claims benefit of Japanese Patent Application(s) No. 2000-9656 filed in Japan on Jan. 19, 2000, the contents of which are incorporated by this reference.
So far, a number of zoom lens systems, each comprising two lens groups or a positive and a negative lens group, wherein the space between them is varied for zooming, have been known as effective arrangements for achieving size and cost reductions and capable of zooming with a simple mechanism. Recently developed zoom lens system are increasingly required to have higher zoom ratios than ever before. Prior zoom lens arrangements to meet such requirements are disclosed in JP-A""s 9-90220, 9-96761, etc.
These arrangements comprising a relatively small number of lenses have a zoom ratio of 2 to 3, and some of them have a zoom ratio of 3 or greater. To reduce the number of lenses used, the second lens group is composed of two lenses or a positive and a negative lens (one of which is an aspherical lens). By making correction for aberrations in the second lens group, performance is maintained all over the zooming zone. However, there is severe degradation of performance due to decentration in the second lens group, because various aberrations are corrected with two lenses. In addition, the power of the second lens group must be increased because the overall negative power of the diverging second lens group is compensated for by the negative lens in the second lens group. This is unfavorable for correction of aberrations.
Referring to JP-A""s 5-119258, 4-22911, etc., a compact yet wide-angle zoom lens system is disclosed. The second lens group is composed of three lenses or a positive, a negative and a negative lens so that various aberrations therein can be corrected. The power of the second lens group is allocated to the three lenses so that the degradation of performance due to decentration can be reduced. However, all three lenses are formed of glass, and so the second lens group is higher in cost than that made up of two lenses. In addition, the back focus is short. This does not only add mechanical constrains to the zoom lens system but also offers several problems such as transfer onto film of dust deposits on the surface of a lens in the vicinity of an image plane, an increase in the diameter of the rear lens, etc. The zoom ratio is far short of 2.
Referring to JP-A 3-267909, etc., the second lens is composed of three lenses or a positive, a negative and a negative lens, with the positive lens being formed of a plastic lens. A plastic lens is excellent in mass productivity, and so has the merit of achieving lower costs as compared with a glass lens. However, a problem with the plastic lens is that its refractive index and shape are prone to large variations depending on ambient temperatures. Accordingly, meticulous care must be taken when the plastic lens is used for a camera""s phototaking optical system. To this end, it is often attempted to make the power of the plastic lens weak. However, such care is not found in the example of JP-A 3-267909 because the power of the plastic lens is still strong.
Referring then to JP-A""s 5-119258, 10-197793, etc., the second lens group is composed of three lenses or a positive, a negative and a negative lens, with the positive lens being formed of a plastic lens, as is the case with JP-A 3-267909. In consideration of the changes of the plastic lens depending on ambient temperatures, the power of the plastic lens is made weak. However, when the power becomes too weak, the effect on correction of aberrations becomes slender. In addition, the principal point positions of the second lens group are shifted to the object side under the power of the second negative lens, resulting in problems such as a decreased back focus. For this reason, how the power of the second negative lens located at a middle position in the second negative lens is determined is important for power profile. The examples show that the power of the second negative lens group is still strong, resulting in a decreased back focus. This does not only add mechanical constrains to the zoom lens system but also offers several problems such as transfer onto film of dust deposits on the surface of a lens in the vicinity of an image plane, an increase in the diameter of the rear lens, etc. When the power of the second negative lens is too weak, on the other hand, the effect on correction of aberrations becomes slender; in other words, the merit of +xe2x88x92xe2x88x92 construction is lost.
In view of such problems associated with the prior art, an object of the present invention is to provide a compact, low-cost zoom lens system of +xe2x88x92 construction, which comprises two lens groups, and an image pickup system using the same.
According to one aspect of the invention, this object is achieved by the provision of a zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and a second lens group having negative refracting power, wherein:
said second lens group comprises, in order from an object side thereof, a positive lens component 2-1, a negative lens component 2-2 and a negative lens component 2-3, with said lens component 2-1 comprising a plastic lens element, and
said second lens group satisfies the following conditions (1) and (2):
1.05xe2x89xa6f21/fT less than 5xe2x80x83xe2x80x83(1) 
3.8 less than f22/fG2 less than 8xe2x80x83xe2x80x83(2) 
where f21 is the focal length of the lens component 2-1 in the second lens group, f22 is the focal length of the lens component 2-2 in the second lens group, fT is the focal length of the zoom lens system at a telephoto end thereof, and fG2 is the composite focal length of the second lens group.
According to another aspect of the invention, there is provided a zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and a second lens group having negative refracting power, wherein:
said second lens group comprises, in order from an object side thereof, a positive lens component 2-1, a negative lens component 2-2 and a negative lens component 2-3, with said lens component 2-1 comprising a plastic lens element, and
said second lens group satisfies the following conditions (1), (2) and (4):
1.05xe2x89xa6f21/fT less than 5xe2x80x83xe2x80x83(1) 
3.8 less than f22/fG2 less than 8xe2x80x83xe2x80x83(2) 
1.01xe2x89xa6SG21 less than 1.24xe2x80x83xe2x80x83(4) 
where f21 is the focal length of the lens component 2-1 in the second lens group, f22 is the focal length of the lens component 2-2 in said second lens group, fT is the focal length of the zoom lens system at a telephoto end thereof, fG2 is the composite focal length of the second lens group, and SG21 is the specific gravity of the lens component 2-1 in the second lens group.
Why the aforesaid arrangements are used in the invention, and how they work is now explained.
According to the present invention, the zoom lens system comprises a first lens group having positive refracting power and a second lens group having negative refracting power. The second lens group then comprises a positive lens 2-1, a negative lens 2-2 and a negative lens 2-3. The positive lens 2-1 is formed of a plastic lens. This arrangement is of the simplest two-group construction in zoom lens constructions, and is constructed of the telephoto type so as to achieve size reductions on the telephoto side. By providing the diverging second lens group of +xe2x88x92xe2x88x92 construction, and especially allocating the high proportion of negative refracting power to two lenses, it is possible to reduce the influence of decentration produced within the second lens group and make correction for aberrations, especially off-axis coma. By constructing the positive lens 2-1 of a plastic lens, size and weight reductions can be achieved.
Referring here to why the plastic lens is used for the lens 2-1 rather than for the lenses 2-2 and 2-3, the lens 2-1 is only the positive lens in the second lens group that has generally negative power, and so can be constructed with a relatively gentle power. In addition, the lens 2-1 is the outermost lens favorable for assembly control. For instance, a plastic lens is fabricated by an injection molding process that does not rely on the centering step needed for glass lenses. This is favorable in consideration of cost, but makes the surface of the lens prone to decentration with respect to the outside shape of the lens. For this reason, it is desired to control the decentration of the plastic lens during assembly. The control should then preferably be carried out with respect to the axes of other lenses forming the same group. This is the reason that the plastic lens should preferably be disposed at the outermost position. How to perform this control, for instance, is set forth in JP-A 6-265766.
Condition (1) provides a definition of the focal length ratio of the lens 2-1 with respect to the zoom lens system at the telephoto end. The outermost lens or plastic lens 2-1 varies in shape and refractive index with temperatures. Such variations occur largely at the telephoto end of the zoom lens system, and have some considerable influences on image-formation capabilities and focal shifts as well. When the lower limit of 1.05 to this condition is not reached, the focal length of the lens 2-1 becomes short (or the refracting power thereof increases strong), resulting in unacceptably large changes of the focal length due to temperature, etc. When the upper limit of 5 is exceeded, the focal length of the lens 2-1 becomes too long to make correction for aberrations, especially chromatic aberrations. This phenomenon becomes perceptible with increasing zoom ratios.
It is here noted that the lower and upper limits to condition (1) may be 1.3 and 3.5, respectively.
Condition (2) provides a definition of the ratio of the focal length of the lens 2-2 with respect to the composite focal length of the second lens group. To satisfy this condition, the combined negative power of the lenses 2-2 and 2-3 must be stronger than the overall negative power of the rear lens group (the second lens group). Basically, positive and negative powers are allocated to the object and image sides of the second lens group, respectively, so that the principal points thereof can be positioned on the object side. By meeting condition (2) in consideration of such requirements, it is possible to ensure the preferable positions for the principal points of the second lens group, and make correction for aberrations of the lens 2-2 in particular. To be more specific, when the lower limit 3.8 to condition (2) is not reached, the proportion of the refracting power of the lens 2-2 in the second lens group becomes large, and so the principal points of the second lens group are shifted toward the object side in the second lens group; that is, the second lens group is as a whole positioned on the image plane side of the zoom lens system. This makes it difficult to ensure any satisfactory back focus. The reduced back focus does not only add mechanical constrains to the zoom lens system but also offers problems such as lens diameter increases, transfer onto film of dust deposits on lens surfaces, etc. When the upper limit of 8 is exceeded, the refracting power of the lens 2-2 becomes too weak to make effective correction for aberrations.
It is here noted that the upper and lower limits to condition (2) may be 5.0 and 7.4, respectively.
By meeting such requirements as mentioned above, it is possible to achieve a compact, low-cost zoom lens system.
In the zoom lens system of such construction as described above, the first lens group comprises, in order from an object side thereof, a front lens unit comprising a negative lens 1-1 and a positive lens 1-2 and having negative refracting power and a rear lens group comprising a positive lens. Preferably in this case, the lens 1-2 is a plastic lens comprising an aspherical surface whose off-axis power is smaller than axial power.
By the wording xe2x80x9caspherical surface whose off-axis power is smaller than axial powerxe2x80x9d used herein is intended an aspherical surface including a surface region wherein, when the axial power is positive power, the off-axis power is smaller than that, and an aspherical surface including a surface region wherein, when the axial power is negative power, the off-axis negative power is stronger than that.
Since the power profile of the first lens group is of the xe2x88x92+ retrofocus type, it is possible to locate the principal points in the first lens group in the rear of the first lens group and so ensure some space between the first and second lens group even at the telephoto end of the zoom lens system. To ensure high zoom ratios, it is essentially required to make good correction for various aberrations within each lens group. However, the first lens group comprises a smaller number of lenses with a large proportion of the positive power allocated to the rear unit, and so the first lens group remains undercorrected. To compensate for this, it is required to use an aspherical surface having negative power that becomes strong at locations off the axis. In consideration of cost, it is preferable to use a plastic aspherical lens because a glass aspherical lens costs much. Since the aspherical surface used is designed to have negative power at locations off the axis, it is preferable to make use of positive paraxial power because fluctuations of focal length with temperature changes can be mutually compensated for within the single lens, so that the fluctuations of focal length with temperature can be reduced.
Preferably, thus constructed zoom lens system should further meet condition (3) given below.
1 less than (R22r+R23f)/(R22rxe2x88x92R23f) less than 2.5xe2x80x83xe2x80x83(3) 
Here R22r is the image-side radius of curvature of the lens 2-2 in the second lens group, and R23f is the object-side radius of curvature of the lens 2-3 in the second lens group.
Condition (3) provides a definition of the shape factor of an air lens formed between the lenses 2-2 and 2-3. When the lower limit of 1 is not reached, it is required to allow an air space between the lenses 2-2 and 2-3, thereby preventing their interference, resulting in an increase in the axial center thickness of the two lens groups and an increase in the thickness of the collapsible mount. In turn, this does not only form an obstacle to size reductions, but also causes the back focus to become short because the principal point positions of the second lens group are shifted toward the object side under the refracting power of the lens 2-2. Exceeding the upper limit of 2.5 to condition (3) means that the air lens defined between the lenses 2-2 and 2-3 takes a meniscus form having close radii of curvature. In other words, at a location off the axis of the air lens, surfaces having close radii of curvature are disposed close to each other. Consequently, light rays reflected at the object-side surface of the lens 2-3, and especially at the periphery of that surface, are reflected at the image-side surface of the lens 2-2. The thus reflected light rays then arrive at an effective screen, yielding ghost or flare components that are harmful to images.
It is here noted that lower and upper limits to condition (3) may be 1.8 and 2.3, respectively.
According to another aspect of the present invention, there is provided a zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and a second lens group having negative refracting power, wherein:
said second lens group comprises, in order from an object side thereof, a positive lens 2-1, a negative lens 2-2 and a negative lens 2-3, with said lens 2-1 comprising a plastic lens element, and
said second lens group satisfies the following conditions (1), (2) and (4):
1.05xe2x89xa6f21/fT less than 5xe2x80x83xe2x80x83(1) 
3.8 less than f22/fG2 less than 8xe2x80x83xe2x80x83(2) 
1.01xe2x89xa6SG21 less than 1.24xe2x80x83xe2x80x83(4) 
where f21 is the focal length of the lens 2-1 in the second lens group, f22 is the focal length of the lens 2-2 in the second lens group, fT is the focal length of the zoom lens system at a telephoto end thereof, fG2 is the composite focal length of the second lens group, and SG21 is the specific gravity of the lens 2-1 in the second lens group.
According to this aspect, too, the zoom lens system comprises a first lens group having positive refracting power and a second lens group having negative refracting power. The second lens group then comprises a positive lens 2-1, a negative lens 2-2 and a negative lens 2-3. The positive lens 2-1 is formed of a plastic lens. This arrangement is of the simplest two-group construction in zoom lens constructions, and is constructed of the telephoto type so as to achieve size reductions on the telephoto side. By providing the diverging second lens group of +xe2x88x92xe2x88x92 construction, and especially allocating the high proportion of negative refracting power to two lenses, it is possible to reduce the influence of decentration produced within the second lens group and make correction for aberrations, especially off-axis coma. By constructing the positive lens 2-1 of a plastic lens, size and weight reductions can be achieved.
Referring here to why the plastic lens is used for the lens 2-1 rather than for the lenses 2-2 and 2-3, the lens 2-1 is only the positive lens in the second lens group that has generally negative power, and so can be constructed with a relatively gentle power. In addition, the lens 2-1 is the outermost lens favorable for assembly control. For instance, a plastic lens is fabricated by an injection molding process that does not rely on the centering step needed for glass lenses. This is favorable in consideration of cost, but makes the surface of the lens prone to decentration with respect to the outside shape of the lens. For this reason, it is desired to control the decentration of the plastic lens during assembly. The control should then preferably be carried out with respect to the axes of other lenses forming the same group. This is the reason that the plastic lens should preferably be disposed at the outermost position. How to perform this control, for instance, is set forth JP-A 6-265766.
Condition (1) provides a definition of the focal length ratio of the lens 2-1 with respect to the zoom lens system at the telephoto end. The outermost lens or plastic lens 2-1 varies in shape and refractive index with temperatures. Such variations occur largely at the telephoto end of the zoom lens system, and have some considerable influences on image-formation capabilities and focal shifts as well. When the lower limit of 1.05 to this condition is not reached, the focal length of the lens 2-1 becomes short (or the refracting power thereof increases strong), resulting in unacceptably large changes of the focal length due to temperature, etc. When the upper limit of 5 is exceeded, the focal length of the lens 2-1 becomes too long to make correction for aberrations, especially chromatic aberrations. This phenomenon becomes perceptible with increasing zoom ratios.
It is here noted that the lower and upper limits to condition (1) may be 1.3 and 3.5, respectively.
Condition (2) provides a definition of the ratio of the focal length of the lens 2-2 with respect to the composite focal length of the second lens group. To satisfy this condition, the combined negative power of the lenses 2-2 and 2-3 must be stronger than the overall negative power of the rear lens group (the second lens group). Basically, positive and negative powers are allocated to the object and image sides of the second lens group, respectively, so that the principal points thereof can be positioned on the object side. By meeting condition (2) in consideration of such requirements, it is possible to ensure the preferable positions for the principal points of the second lens group, and make correction for aberrations of the lens 2-2 in particular. To be more specific, when the lower limit 3.8 to condition (2) is not reached, the proportion of the refracting power of the lens 2-2 in the second lens group becomes large, and so the principal points of the second lens group are shifted toward the object side in the second lens group; that is, the second lens group is as a whole positioned on the image plane side of the zoom lens system. This makes it difficult to ensure any satisfactory back focus. The reduced back focus does not only add mechanical constrains to the zoom lens system but also offers problems such as lens diameter increases, transfer onto film of dust deposits on lens surfaces, etc. When the upper limit of 8 is exceeded, the refracting power of the lens 2-2 becomes too weak to make effective correction for aberrations.
It is here noted that the upper and lower limits to condition (2) may be 5.0 and 7.4, respectively.
Condition (4) provides a definition of the specific gravity of the plastic lens 2-1. As already explained, a plastic lens can contribute to weight reductions because of being smaller in specific gravity than a glass lens. With size reductions of a camera, weight reductions of lenses, too, provide effective means for saving the power and energy of a built-in motor.
With the second embodiment of the present invention, too, a compact, low-cost zoom lens system can be achieved by meeting such requirements as mentioned above.
Preferably, the front lens unit in the first lens group should consist of, in order from an object side thereof, a negative meniscus lens element and a positive meniscus lens element convex on an object side thereof.
Preferably, the rear lens unit in the first lens group should consist of a positive double-convex lens component.
Preferably, the second lens group should consists of, in order from an object side thereof, a positive meniscus lens element concave on an object side thereof, a negative lens element concave on an object side thereof and a negative meniscus lens element concave on an object side thereof.
It is thus possible to construct a high-performance zoom lens system of a reduced number of lenses.
Preferably in view of processability and correction of aberrations, aspherical surfaces should be used at the object-side surface of the lens component 1-2 in the first lens group and the object-side surface of the lens component 2-1 in the second lens group.
When an aspherical surface is used at the object-side surface of the lens component 1-2 in the first lens group, it should preferably have positive power on the optical axis, and be configured in such a way as to have a point of inflexion on section including the optical axis.
Preferably, a stop designed to move together with the first lens group during zooming should be disposed between the first and second lens groups.
Upon zooming from the wide-angle end to the telephoto end of the zoom lens system, both the first and second lens groups should preferably move toward the object side of the zoom lens system with a varying separation between them.
Of groups comprising lenses, only the first and second lens groups should preferably move upon zooming from the wide-angle end to the telephoto end, with a zoom ratio of 2.5 or greater. More preferably, the zoom ratio should be 3.1 or greater.
According to a further aspect of the present invention, the zoom lens system of the present invention may be used as an image pickup device to construct an image pickup system comprising a viewing device for viewing an image formed by the zoom lens system.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.